The use of micro-electromechanical sensors (MEMS), rather than more traditional optically-tracked sensors for example, for the purposes of communicating with a computer-assisted surgery (CAS) system such as to navigate tools, bones, reference markers, etc. within the surgical field is becoming more desirable because such MEMS sensors are not limited by line-of-sight requirements of previously used optical sensors. These MEMS, which may for example include accelerometers and/or gyroscopes, are able to wirelessly communicate with the CAS system with which they are employed, or are equipped with a processor and user interface providing guidance to a user, i.e., the MEMS unit is part of a portable CAS system that is directly on the CAS instrument. Accordingly, the CAS system is able to determine at least orientation information of the MEMS unit, and therefore able to locate and track (i.e. navigate) the tool or bone to which the MEMS unit is fastened.
One of the steps required to navigate any tracked bone reference or surgical tool using a CAS system, including one which employs MEMS, is to “calibrate” the CAS instrument (ex: either a bone reference or tool) by precisely locating the position and/or orientation of the sensor relative that of the CAS instrument to which it is fastened. In the event that the sensor is detached from the CAS instrument, it is typically necessary to re-calibrate the assembly once the sensor is again fixed in its normal position relative to the instrument.
It may be desirable to be able to switch MEMS from one CAS instrument to another, however this becomes problematic because each time the MEMS is removed and then re-fastened, either onto another instrument or even back onto the same original instrument from which it was detached, a new calibration step must be performed in order to ensure that the exact relative position of the MEMS and the CAS instrument to which it is fastened are determined by the CAS system.
Accordingly, there is a need for a MEMS-navigated CAS instrument which enables the MEMS to be readily removed and re-attached to the instrument with repeatable precision and accuracy such that the relative alignment and orientation of the instrument and the MEMS will remain constant, thereby avoiding the need to re-calibrate the entire instrument in the event that the MEMS sensor is removed and/or re-attached.